Formation of corrosion-resistant coating

ABSTRACT

A coating process comprising: (A) applying to a surface, for example, a metallic surface, a coating compositions consisting essentially of an alkali metal silicate and an aqueous liquid phase having dispersed therein solid aluminum particles to form on the surface a wet coating; and (B) drying said wet coating : (I) under conditions which convert said wet coating to an electrically conductive, corrosion-resistant, solid coating; or (ii) under conditions which form a solid coating which is not electrically conductive (non-conductive) and thereafter treating said non-conductive coating under conditions which convert said non-conductive coating to an electrically conductive, corrosion-resistant coating compositions for use in the process, and the provision of highly corrosion-resistant coated articles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. application Ser. No.60/416,575, filed Oct. 7, 2002.

FIELD OF THE INVENTION

The present invention relates to the formation of a corrosion-resistantcoating on a metallic surface. More particularly, the present inventionrelates to a silicate coating composition, to its use to form on ametallic surface a coating which is highly corrosion-resistant, and to acoated article having thereon a silicate coating which is highlycorrosion-resistant.

The present invention will be described initially in connection with itsuse to form highly corrosion-resistant coatings on the surfaces ofturbine engines, for example, airplane turbine engines and gas- orsteam-powered ground turbine engines. It should be understood, however,that the present invention can be used also in other applications, aswill be evident from the detailed description of the invention whichappears below.

The operation of a turbine engine generates very high temperatures towhich various parts of the engine are exposed. For example, thetemperature in the combustion chamber of the engine can reach 2200° F.or higher. Other parts of turbine engines which are subjected to suchhigh temperatures include, for example: stators; blades; discs; turbineshafts; and exhaust ducts.

The housing of a turbine engine and parts comprising the engine are madetypically from specialty steels, for example, stainless steel and fromhigh-strength, light-weight, titanium-based alloys. As is well known,iron-based metals tend to corrode (rust) in the presence of water andare weakened structurally as the rusting process progresses. Turbineengines and the parts thereof typically come into contact with water,for example, as moisture condenses on the various surfaces of the enginewhen the engine is not in use. Also, airplane turbine engines can comeinto contact with salt water which has a highly corrosive effect oniron-based parts. If the various surfaces of the turbine engine are notprotected from contact with water, there can be engine failure as one ormore of the parts lose strength due to the corrosive effect of thewater.

Accordingly, it is well known to apply to the various surfaces ofturbine engines coatings which protect the underlying metallic surfacesfrom contact with water and which function as corrosion-resistantcoatings. Such coatings must withstand, of course, the high operatingtemperatures of the engine.

At the high operating temperatures of the engine, the various metallicsurfaces of the engine, including those comprising iron-based andtitanium-based alloys, are subject also to being oxidized, a reactionknown as “heat oxidation”. Parts of the engine which are vulnerable toheat oxidation include the combustion chamber, the power turbine and theexhaust chamber. Heat oxidation can result also in engine failure asparts of the engine are weakened structurally. Accordingly, thecorrosion- and heat-resistant coating should function also to protectcritical underlying metallic surfaces from being oxidized in thepresence of the large amounts of heat generated by the operation of theengine.

For industrial acceptance, the coatings for use in such turbine engineapplications must have also a combination of other properties,including, for example, flexibility properties, crack-resistantproperties, hydraulic oil-resistant properties, and abrasion-resistantproperties.

The present invention relates to the provision of a coating compositionwhich is capable of forming on a metallic surface a highlycorrosion-resistant coating that has a combination of properties of thetype required for successful use in industrial turbine engineapplications as well as other industrial coating applications.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a coatingprocess comprising: (A) applying to a surface an aqueous coatingcomposition consisting essentially of an alkali metal silicate andhaving dispersed therein solid aluminum particles to form on the surfacea wet coating; and (B) drying said wet coating under conditions whichconvert said wet coating to an electrically conductive, solidcorrosion-resistant coating or drying said wet coating under conditionswhich form a solid coating which is not electrically conductive(non-conductive) and thereafter treating said non-conductive coatingunder conditions which convert said non-conductive coating to anelectrically conductive, corrosion-resistant coating.

In preferred form, the coating composition is substantially free ofchromium. Also, in preferred form, the aluminum particles are dispersedin an aqueous solution of the alkali metal silicate. In addition, thepreferred form of the coating composition includes one or more additiveswhich are effective in improving the corrosion-resistant properties ofthe coating. An organic solvent and a silane are examples of suchadditives.

Another aspect of the present invention is the provision of a surfacecoated with an electrically conductive silicate coating comprisingaluminum. In preferred form, a metallic surface is provided with acoating that has a thickness of about 0.8 mil to about 3.5 mils andcorrosion-resistant properties characterized by no greater than about1.6 mm loss of adhesion at scribe when subjected to 5% neutral saltspray at 95° F. for at least about 1000 hours (ASTM B-117).

There are numerous advantages that are associated with the presentinvention. Chromium-based coating compositions have been considered formany years as the standard in industry for forming coatings which arehighly corrosion-resistant. The present invention enables one to formsuch highly corrosion-resistant coatings, but without the need to useenvironmentally detrimental constituents like hexavalent chromium.Another advantage of the composition of the present invention is that itis a “one-part” composition in that all of the constituents can be mixedtogether into a single formulation well prior to use and without one ormore of the constituents affecting adversely other constituents of thecomposition. Non-chromium-based compositions of the prior art aretypically “two-part” compositions which need to be mixed together justprior to use. Other advantages of the present invention are discussedbelow.

It is believed that the present invention will be used widely to coatand protect various types of surfaces, particularly the metallicsurfaces of a turbine engine, including the housing and various parts ofthe engine.

DETAILED DESCRIPTION OF THE INVENTION

As set forth above, the present invention includes within its scope theprovision of an electrically conductive, silicate coating which includesaluminum, which is highly corrosion-resistant, and which has otherdesirable properties, as discussed in detail below. As discussed belowalso, various of the desired properties of the conductive silicatecoating are much better than those of a silicate coating which isnon-conductive. The term “electrically conductive” means that thecoating has an ohm value of no greater than about 20, preferably nogreater than about 15, and more preferably less than about 10, asdetermined by the conductivity test which is described below in the textof Example 1 hereof.

The silicate component of the coating functions as a film-former andbinder which binds together the other constituent(s) of the coating andthe coating to the underlying substrate. The aluminum constituent of thecoating imparts thereto sacrificial corrosion properties, that is, thealuminum reacts preferentially with materials which would tend to reactwith the underlying substrate and cause degradation thereof. This“sacrificial property” deters corrosion of the underlying substrate.

The electrically conductive silicate coating can be formed on theunderlying substrate in any suitable way. In accordance with the presentinvention, it is recommended that the coating be formed initially as awet coating from a liquid composition that contains an alkali metalsilicate, aluminum particles, and water and that the wet coating bedried under conditions which convert the wet coating to an electricallyconductive solid form or that the wet coating be dried under conditionswhich form a non-conductive solid coating which is then converted to anelectrically conductive form. It should be appreciated that theconventional use of the aforementioned type of liquid silicate coatingcomposition results in the formation of a coating which isnon-conductive and which has properties, including corrosion-resistantproperties, which are substantially poorer than the conductive form ofthe coating.

The corrosion-resistant coating of the present invention can be formedfrom an aqueous coating composition comprising an alkali metal silicateand aluminum particles dispersed therein. The composition can includealso optional ingredients, as described below. As mentioned above, oneof the advantages associated with the use of the composition is that itis not necessary to keep one or more of the constituents comprising thecomposition separated from another of the constituent(s) just prior tothe time the composition is to be used. Other types of prior artcompositions that have been used industrially to coat turbine engineparts include components which have a short “pot-life”, that is, uponbeing mixed, the resulting composition has to be used in a relativelyshort period of time (for example, within about 1 hour to about 6 hours)or it becomes unusable for the coating application. Preferredcompositions for use in the present invention are stable and indeed havea long shelf-life, for example, about 10 months or longer.

Alkali metal silicates are well known materials which are available inliquid form or solid form, for example in, powdered form. Any suitablealkali metal silicate can be used in the composition of the presentinvention. Examples of alkali metal silicates are sodium silicate,lithium silicate, and potassium silicate. A mixture of two or morealkali metal silicates may be used also. Preferred alkali metalsilicates are sodium silicate and lithium silicate. It is preferredparticularly to use a mixture of sodium silicate and lithium silicate.

Sodium silicate is used preferably in liquid form, for example, as anaqueous solution of glasses made by fusing varying proportions of sandand soda ash. The proportions of sand and soda ash that are useddetermine the SiO₂:Na₂O weight ratio of the sodium silicate. Forexample, there are available commercially liquid (water-based) sodiumsilicates that have a SiO₂:Na₂O weight ratio of about 1.6:1 to about3.75:1 and that have viscosities which range from those of a syrupyliquid (for example, 1.8 poises at 20° C.) to a thick alkaline liquid(for example, 700 poises at 20° C.). A preferred liquid silicate has aSiO₂:Na₂O weight ratio about 2.5:1 to about 3.2:1.

Examples of commercially available liquid sodium silicates include thosesold by The PQ Corporation under the registered trademarks: “STIXSO RR”;N; E; O; K; M; RU; D; C; and STAR. Examples of powdered sodium silicatesare those sold by The PQ Corporation under the registered trademarks“SS” 65 pwd; “G”; “GA”; and GD. Commercially available sodium silicatesinclude those which comprise about 9 to about 27 wt. % Na₂O and about 20to about 75 wt. % SiO₂.

Any suitable lithium silicate can be used in the composition. Thelithium silicate can be in solid or liquid form and have, for example, aSiO₂:Li₂O weight ratio of about 9.4:1 to about 17:1, with the preferredratio being about 9:1 to about 10:1. Examples of commercially availablelithium silicates include Ludox® lithium polysilicate.

Any suitable potassium silicate can be used in the composition. Thepotassium silicate can be in solid or liquid form and have, for example,a SiO₂:K₂ ₂O weight ratio of about 1.6:1 to about 2.5:1. Examples ofcommercially available liquid potassium silicates those sold by The PQCorporation under the registered trademark KASIL.

The alkali metal silicate should be used in the composition in an amountat least sufficient to form a continuous adherent coating on the surfaceof the substrate and to bind the aluminum particles which are includedin the coating. The maximum amount of alkali metal silicate comprisingthe coating composition is dictated by the ability to bond the aluminumparticles to the surface without the coating's blistering or mudcracking as it is cured. It is believed that the most widely usedcompositions will comprise about 2.5 to about 30 wt. % of the alkalimetal silicate. Preferably, the composition comprises about 7 to about13 wt. % of the alkali metal silicate.

In the use of a mixture of sodium silicate and lithium silicate, it isrecommended that the sodium silicate:lithium silicate weight ratio beabout 0.25:1 to about 4:1, with the preferred ratio being about 0.6:1 toabout 1.5:1. In the use of a mixture of sodium and lithium silicates, itis recommended that the composition comprise a total silicate content ofabout 2.5 wt. % to about 30 wt. %, with the preferred amount being about7 wt. % to about 13 wt. %.

The composition of the present invention includes also solid aluminumparticles, for example, in the form of flake, powder, or granules. It ispreferred that the aluminum particles be in the form of a powder. Thealuminum particles should be of a size sufficiently small to enable theparticles to be dispersed in the liquid composition, perferablyuniformity throughout the composition. For dispersiblity, it ispreferred that the average size of the aluminum particles be no greaterthan about 15 microns. Typically, the average size of the aluminumparticles should be about 2 microns to about 10 microns. A particularlypreferred average particle size is about 4 microns to about 7 microns.Examples of commercially available aluminum particles include ToyalAmerica 105 and Toyal America 5662.

The aluminum particles should be used in the composition in an amountsuch that the coating can be made conductive. The maximum amount ofaluminum particles comprising the composition is governed bysprayability considerations and coating defects which tend to beencountered if too much aluminum is used, loss of adhesion and surfacedefects in the coating such as mud cracking. It is believed that themost widely used compositions will comprise about 20 to about 50 wt. %of the aluminum particles. Preferably, the composition comprises about35 to about 45 wt. % of the aluminum particles.

Optional materials can be included in the aqueous composition in amountseffective to achieve desired effects. Examples of optional materials arewetting agents, phosphates, fluorocarbons, polysiloxanes, waterrepellants, rheology modifiers, and nanopowders.

Any suitable wetting agent can be used in the composition. The wettingagent should function to modify the surface characteristics of thesubstrate being coated in a manner such that the uniform application ofthe water-based coating composition is more readily achieved and thetendency of surface defects to form in the coating is reduced. Examplesof suitable wetting agents that can be used and include anionic,nonionic, cationic, and amphoteric wetting agents. Preferred classes ofwetting agents are silanes, fluoropolymer type wetting agents,polysiloxanes, and phosphates. Preferred species of wetting agents areLodyne S222 fluorocarbon, Byk 348 polysiloxane, Zonyl FSN fluorocarbon,Coat-O-Sil 1211 silane, Cirrasol G-2200 alkyl phosphate. The wettingagent can comprise about 0.05 wt. % to about 1 wt. %, preferably about0.05 wt. % to about 0.2 wt. % of the composition.

Addition of a phosphate containing compound (either organic orinorganic) may be used to improve various coating properties, forexample, adhesion to the underlying substrate and flexibility. It hasbeen observed, however, that the presence of a phosphate may improvecertain properties at the expense of affecting adversely otherproperties. For example, flexibility of the coating can be improved bythe use of phosphate in the coating composition, but a decrease incorrosion-resistance can be experienced. Examples of sources ofphosphate, which include water soluble phosphates, are trisodiumphosphate, sodium tripolyphosphate, ferric pyrophosphite, sodiumpyrophosphate (particularly preferred), ammonium phosphate, and tributylphosphate (preferred). Typically, the phosphate can comprise about 0.1%to about 2.5 wt. % of the total composition, with the preferred rangebeing about 0.2% to about 1 wt. %.

Including in the coating composition a compound which is generallyreferred to in the art as an “organic solvent” can reduce or prevent theformation of surface blisters in the coating. The formation of surfaceblisters has been observed, for example, as the corrosion-resistantproperties of multi-ply coatings have been evaluated in salt spraytests. Blisters have an adverse effect on the corrosion-resistantproperties of the coating and their formation in coatings used inindustrial applications would be undesirable. It has been observed alsothat the use of the organic solvent improves the ability of thecomposition to be applied more readily and uniformly to the substrateand to form smooth coatings.

The organic solvent is a liquid at room temperature and has surfaceactive properties, but differs from a wetting agent, as described above,in that the solvent is 100% volatile and is capable of dissolvinganother substance and, in some cases, can be used to help dissolve awetting agent when used and as needed. The organic solvent is alsotypically used at a higher percentage compared to a wetting agent. Theorganic solvent can be used, for example, at a level of 2% by weight orhigher for beneficial effects, whereas the effectiveness of a wettingagent can be realized at concentrations of 1% or lower. The organicsolvent should be a compound which is compatible with the otherconstituents of the aqueous coating composition. For example, theaddition of the solvent to the coating composition should not causeprecipitation of the silicate constituent or other constituents of thecomposition. A preferred group of organic solvents for use in thepresent composition comprises a solvent which is partially miscible inwater, that is, the solvent has a miscibility in water of about 1 ml toabout 20 ml of solvent per 100 ml of water at about 20° C. An aqueouscomposition which includes an organic solvent that has a lower degree ofmiscibility with water is evidenced by the formation of a layer of asolution of the water and organic solvent (the miscible layer) and alayer of the organic solvent. More preferably, the partiallywater-miscible organic solvent is miscible in water up to about 10 mland, most preferably, up to about 5 ml of solvent per 100 ml of water atabout 20° C. The solvent may have a miscibility of about 0.1 ml/100 mlof water (or even lower) at 20° C. Accordingly, the solvent may beimmiscible in water.

Partially water-miscible organic solvents for use in the practice of thepresent invention are liquid aliphatic and aromatic carbon compoundswhich have typically a hydrophilic group, for example, an ether group,most typically a hydroxyl (—OH) group. Examples of classes of compoundswhich include such solvents are glycols, glycol ethers, ketones, esters,and alcohols. A glycol ether is a preferred class of compounds, forexample, propylene glycol n-butyl ether. A particularly preferred glycolether is dipropylene glycol n-butyl ether.

The organic solvent should be included in the composition in an amountsufficient to reduce the formation of blisters in the coating in thoseapplications in which they tend to form. It is believed that, for mostapplications, the amount of solvent will fall within the range of about0.5 to about 10 wt. % of the composition. Preferably, the compositioncomprises about 4 to about 6 wt. % of the solvent.

It is theorized that mechanisms involved in the functioning of thesolvent to reduce blister formation are as follows. The presence of thesolvent in the composition is believed to change the rate of evaporationof the water constituent; this causes the silicate to polymerize andprecipitate (solidify) in a different manner than when the solvent isnot present. This, in turn, leads to the formation of coatings that aremore resistant to being degraded by high moisture conditions. It isbelieved also that the solvent functions to wet both the surfaces of thesubstrate being coated and the aluminum particles; this aids in theformation of a cured coating which has improved bonds that tightlyadhere to the underlying surface and retain their integrity, even in thepresence of high moisture conditions.

Another additive that can be included in the coating composition toreduce or prevent the formation of surface blisters in the coating is anorganofunctional silane. It has been observed that the use of such asilane improves also surface wetting, adhesion, and moisture-resistancein the cured coating. The silane can be used in admixture with theorganic solvent.

Many species of organofunctional silanes are known. For example, thefollowing publications disclose species of such silanes and contain alsoa substantial amount of information on organofunctional silanes: (A)OrganoSilicon, Products—Systems—Services, Product Information, UnionCarbide Organofunctional Silanes for Coatings, SC-1603B, Union CarbideCorporation, Specialty Chemicals, Danbury, Conn. (1993); (B) Silquest®Silanes Products and Applications, Witco Corporation, Greenwich, Conn.;(C) Silquest Organofunctional Silanes for Waterbome Systems, AdhesionPromoters and Crosslinkers, OSi Specialites, Inc.;(D) Silquest A-1123Silane, Low Chloride Adhesion Promoter and Crosslinker, OSi Specialties,Inc.; and (E) Organofunctional Silanes, PO-2266, SC-1294, December 1991,OSi Specialties, Inc.

Speaking generally, an organofunctional silane comprises a functionalmoiety that contains an Si atom and an organic moiety connected to theSi atom. The functional Si-containing moiety is capable of hydrolyzingin the presence of water to form a silanol (—Si(OH)_(n)) which in turnis capable of reacting with, for example, reactive sites on inorganicsurfaces, for example, metallic surfaces and surfaces of pigmentparticles. The silanol is capable also of co-condensing to effectcrosslinking of polymers through a moisture-cure mechanism.

Organofunctional silanes can be grouped into two classes, namely,organoreactive organofunctional silanes and non-organoreactiveorganofunctional silanes (hereafter, for convenience “organoreactivesilanes” and “non-organoreactive silanes” respectively). These classesare described hereafter.

The organoreactive silane contains in its organic moiety a functionalgroup that is capable of reacting with one or more functional groups ona polymer or on a monomer for use in polymerization. Examples ofreactive functional groups that are present in organoreactive silanesare vinyl, methacryl, epoxy, mercapto, amino, ureido, and isocyanato.The organoreactive silanes are described for use in a variety ofapplications, for example, in polymer synthesis as chain transfer,end-blocking, and crosslinking agents. In addition, they are known to beused in polymer-based coating or paint compositions where they combinechemically with the polymeric binder of the composition to improve avariety of properties in coatings formed from the compositions. Suchproperties include, for example, coating strength, adhesion, durability,weather-resistance, scrub-resistance, and mar- and abrasion-resistance.

The non-organoreactive silane has an organic moiety which isnon-reactive with constituents that are present in the environment ofits use. Non-organoreactive silanes are reported as being useful inimproving the dispersion of pigment and filler and various properties ofcoating compositions in which they are used and coatings formed from thecoating composition. Examples of such properties include ease of mixingand improved gloss, hiding power, and water-resistance of coatingsformed from compositions that contain the silane.

A silane for use in the practice of the present invention can berepresented by the formula R_(n)—Si(X)_(4-n) in which R representseither an organic moiety that contains a reactive functional group, asdescribed above, or an organic moiety that is not reactive, as describedabove. X represents an alkoxy group, for example, methoxy, ethoxy, andacetoxy. Examples of reactive functional groups that can be included inthe organic moiety (R) are identified above in connection with thegeneral description of the organoreactive silanes. Examples of theorganic moiety (R) in the non-organoreactive silanes are alkyl andphenyl.

Silanes for use in the practice of the present invention can berepresented also by the following formula:Y—(CH₂)_(n)—SiX₃in which: Y is an organic moiety that is attached to the silicon atom bya stable (CH₂)n carbon chain and that contains a reactive group, forexample, —Cl, —NH₂, —SH,

X is a functional group that is capable of hydrolyzing and reacting withactive sites on inorganic surfaces, for example, —OCH₃, —OC₂H₅, and—OC₂H₄OCH₃.

Examples of species of nonorganoreactive silanes includehexadecyltrimethoxysilane and methyltriethoxysilane.

The preferred silane for use in the practice of the present invention isan organoreactive silane. In particularly preferred form, theorganoreactive silane includes in its organic moiety one or more of anamino or epoxy group, for example, gamma-aminopropyltriethoxysilane.Particularly preferred species of organoreactive silanes areN-beta-(aminoethyl)-gamma-aminopropyltrimethoxysilane andgamma-glycidoxypropyltrimethoxysilane.

The organofunctional silane should be included in the composition in anamount sufficient to reduce the formation of blisters in the coating inthose applications in which they tend to form. It is believed that, formost applications, the amount of silane will fall within the range ofabout 0.1 to about 5 wt. % of the composition. Preferably, thecomposition comprises about 1 to about 3 wt. % of the silane.

It is theorized that mechanisms involved in the functioning of thesilane to reduce blister formation are as follows. It is believed thatthe silane functions to make the coating more insoluble to water in thatit causes the silicate to precipitate in a different manner than whenthe silane is not present. It is believed also that the silane aids inthe wetting of the substrate and the aluminum particles. The silane mayact also as a coupling agent, helping to bond the coating to thesubstrate, as well as reacting onto the backbone of the silicatestructure to form a less water-soluble, silicate-based polymer.

The amount of water comprising the aqueous-based silicate composition isgenerally dictated by viscosity needs, sprayability, and the amountnecessary to allow the coating to cure without blistering. For mostapplications, it is believed that the water content of the compositionwill comprise about 35 to about 70 wt. % of the composition, with anamount of about 45 to about 55 wt. % being preferred.

An important advantage of the present invention is that the pH of theaqueous-based silicate composition is such that it does not degrade thealuminum particles or the metallic surface, for example, mild steels,which tend to be attacked or degraded by other types of coatingcompositions that are relatively acidic. The pH of the coatingcomposition is dictated generally by the amount of silicate in thecomposition and the SiO₂:M₂O ratio. Typically, the pH of the compositionwill be about 10 to about 14, with a pH of about 11.5 to about 12.5being preferred.

As mentioned above, the silicate coatings of the present invention canbe formed from a coating composition that comprises environmentallyacceptable constituents. Accordingly, the coatings of the presentinvention are capable of being formed from coating compositions which donot contain hexavalent chromium.

The coating composition can be applied to the surface of the substratebeing coated in any suitable way, for example, dipping, spraying,brushing, and rolling. The amount of composition applied to the surfacewill depend on the thickness of the coating to be formed on thesubstrate. An exemplary coating thickness is about 1 to about 3 mils.

After the liquid coating composition is applied to the substrate, theresulting liquid coating should be allowed to dry and solidify (cure).This involves typically the evaporation of the water constituent of thecomposition and can include also a chemical setting mechanism, forexample, treating the coating with ZnO, CaO, or an acidic wash. Curingthe wet coating utilizing conventional curing methods, for example, bydrying the wet coating at room temperature or at a relatively lowelevated temperature, for example, about 250 to about 600° F. results ina solidified coating that is not electrically conductive.

In one embodiment of the invention, an electrically conductive coatingcan be obtained directly by curing the wet coating at a relatively hightemperature. The particular temperature used will depend on variousfactors, including, for example, the thickness of the coating, theamount of aluminum in the coating, and type of aluminum, and theparticle size of the aluminum. Also, the lower the temperature, thelonger the coating needs to be subjected to the elevated temperature.From a practical standpoint, the curing temperature should be at leastabout 950° F. For guideline purposes, it is noted that electricallyconductive coatings have been achieved by curing the coating at atemperature of about 1000° F. for about 1 hour. Higher temperatures canbe used. The maximum curing temperature is dictated typically by thetemperature at which the aluminum will change to aluminum oxide in air.It is believed that at such elevated temperatures and curing conditions,the aluminum particles of the coating expand and the expanded particlescontact one another more intimately and to the extent that theelectrical conductivity of the coating is improved and converted to aconductive coating, as defined herein.

Preferably, multi-stage curing conditions are used, that is, curing iseffected initially for a time and at a temperature which acceleratesevaporation of the water in the coating, but which does not result inany loss of the water of hydration of the alkali metal silicate, forexample, for about 15 minutes at 175° F. After the coating has cured tothe extent that all of the free water has evaporated, the temperaturecan be raised to accelerate the cure further, such temperature being,for example, about 1000° F. and for a period of time of, for example,about one hour. The multi-stage curing deters the formation of coatingdefects which tend to be formed as a result of the surface of thecoating's curing before all free water has been released.

In a preferred embodiment of the invention, an electrically conductivecoating is obtained indirectly. This involves curing the wet coating ata relatively low temperature, for example, about 400 to about 650° F.for about ½ hour to about 1 hour to form a solid coating that is notelectrically conductive. In a particularly preferred embodiment,multi-stage curing conditions are used, for example, an initial stageinvolving drying at a temperature of about 175° F. for 15 minutes andthereafter at 600° F. for about 30 minutes. Such non-conductive coatingcan be converted to an electrically conductive coating by subjecting thecoating to conditions which are effective in compressing the coating todecrease the distance between the aluminum particles and force theparticles into more intimate contact with one another and with thesubstrate. This can be accomplished, for example, by peening orburnishing the non-conductive coating. Burnishing the coating ispreferred, for example, by blasting the coating with a material which iseffective in performing said compression. Examples of such materials areAL203 grit (240 mesh), glass beads, and any other suitable media whichis used in commercial blasting equipment. Burnishing the coating may beachieved also by tumbling or vibrating the coated article in thepresence of a material which is effective in performing said compressionand/or peening, for example, ceramic beads, other forms of ceramic, andsteel media. Subjecting the coating to compressive and/or peening forcesshould be carried out for a period of time sufficient to convert thecoating to an electrically conductive form. Exemplary time periods forachieving this are about 30 seconds to about 30 minutes.

The coating composition of the present invention can be used to formcorrosion-resistant coatings on any suitable surface including, forexample, metallic surfaces such as stainless steel, low-grade steel, andother iron alloys, titanium-based alloys, and aluminum and aluminumalloys. It is believed that the present invention will be used widely'toform corrosion-resistant coatings on the surfaces of parts of airplaneand ground turbine engines. Other exemplary applications for the use ofthe present invention are the coating of fasteners, exhaust headers,turbochargers, other engine components that are subjected to hightemperatures, heat exchangers, and burner components. An example of anon-metallic surface on which the electrically conductive coating can beformed is a ceramic surface.

The thickness of the coating that is formed on the surface of thearticle should be at least sufficient to form a coating that has thedesired corrosion-resistant properties. It is believed that, for mostapplications, the thickness of the coating will be about 1 mil to about4 mils. For applications which involve a higher degree of corrosionprotection, it is recommended that the coating thickness be about 2 toabout 3.5 mils.

Multiple coats of the coating composition can be applied to thesubstrate to form a multi-ply coating, typically a two-ply coating. Informing a multi-ply coating, the underlying ply can be converted to anelectrically conductive form prior to applying the overlying coating orthe underlying coating can be left in its non-conductive form and coatedwith the overlying coating. In each of such embodiments, the overlyingcoating is treated in accordance with the present invention to convertit into an electrically conductive coating.

There have been circumstances where the formation of surface blistershas been encountered in the production of a multi-ply coating. (Blisterformation has been observed in a multi-ply coating which has been driedand/or cured at elevated temperatures, as described above, and which hasbeen subjected to salt spray tests for evaluation.) In such acircumstance, the following is a recommended procedure. After applyingan underlying layer of wet coating composition to the substrate, the wetlayer is air-dried before the application thereto of an overlying layerof coating composition, that is, the underlying layer of wet coatingcomposition is not subjected to elevated temperature(s) to acceleratethe drying and/or curing thereof. After applying to the air-driedunderlying coating a layer of the overlying coating, the resultantmulti-ply coating is subjected to elevated temperature(s) to dry andcure the multi-ply coating in the manner described above. This appearsto cause the underlying layer of air-dried coating and overlying layerof wet coating to fuse together and cure at the same time. The formationof blisters in the overlying cured coating can be prevented by followingthis procedure.

In preferred form, the coating is formed on a grit blasted clean ironalloy surface and has a thickness of about 0.8 mil to about 3.5 mils andcorrosion-resistant properties characterized by no greater than about1.6 mm loss of adhesion at scribe when subjected to 5% neutral saltspray at 95° F. for about 1000 hours (ASTM B-117).

EXAMPLES

The following is an example of a coating composition for use in thepresent invention. Unless indicated otherwise, “%” means weight percentbased on the total weight of the composition.

Example No. 1

Constituents Wt. % sodium silicate, wt. ratio SiO₂:Na₂O - 3.22 11.6aluminum powder, average particle size 4.5 microns 43 sold by ToyalAmerica as No. 105 water 45.4The source of the sodium silicate was a thick liquid (viscosity of 20°C.-4.0 poises) which is sold by The PQ Corporation under the trademark“O” and which comprises 9.15% Na₂O, 29.5% SiO₂, and 61.35% water.Additional water was added to the liquid sodium silicate to adjust thetotal water content of the composition fo 45.4% and the resultingmixture was stirred for about 5 minutes to mix completely the sodiumsilicate. Next, the aluminum powder was added to the mixture and stirredtherein for about 10 minutes to form 19 ml of an aqueous silicatesolution having dispersed uniformly therein the aluminum powder. Theresulting coating composition was applied to 1010 steel panels(3″×5″×0.03″) by spray application with conventional air-spray equipmentuntil a uniform wet coating of the desired thickness was obtained.

The wet coating formed from the coating composition was then air drieduntil dry to the touch and then further cured at a temperature of 175°F. for 15 minutes followed by 600° F. for 30 minutes. Curing of the wetcoating resulted in a solid coating which had a thickness of 2 mils andwhich was determined to be non-conductive in that it has an ohm readingof greater than 20 ohms. The conductivity (or lack thereof) of the curedcoating was determined by measuring the resistance of the coating inohms using an ohmmeter with 2 blunt probes. The 2 probes are lightlyplaced one inch apart on the cured coating so as not to penetrate thesurface of the coating. A reading of no greater than about 20 ohms isconsidered conductive.

The non-conductive coating (greater than about 20 ohms) was burnished inorder to convert it into a coating that was electrically conductive byburnishing for about 1 minute with 240 mesh aluminum oxide grit at 40psi in a suction blast cabinet. The conductivity of the burnishedcoating was measured and determined to have an ohm reading of 2.5 ohms.Accordingly, the coating was conductive. The conductivity of theburnished coating was evaluated in the same way as that of theaforementioned non-conductive coating.

The electrically conductive coating was then evaluated for:corrosion-resistance; adhesion; flexibility; abrasion-resistance; andhydrolytic stability. The tests used to evaluate such properties aredescribed below:

-   -   (A) corrosion-resistance: ASTM B-117 salt spray test involving        placing a coated article having an “x” scribed on the coating        into an environmental- and temperature-controlled chamber and        subjecting the coated article to a 5 wt. % neutral NaCl salt        water spray at 95° F. for a predetermined number of hours;    -   (B) adhesion and flexibility: bend test involving bending a        coated metal panel at 90° around a ¼ inch mandrel, then        attempting removal of the coating by applying 3M #250 tape at        the bend and thereafter removing the tape quickly;    -   (C) abrasion-resistance: ASTM D968-81 falling sand test        involving dropping sand onto a coated article at a rate of 2        liters of sand in 21 to 23.5 seconds; and    -   (D) hydrolytic stability: boiling water test involving immersing        a coated metal panel in boiling water for a period of 10        minutes, removing it from the water, allowing it to air dry and        cool for at least 1 hour at room temperature, and then        subjecting it to the above bend test.

The results of testing various samples of panels coated as describedabove are set forth below. Properties Evaluated Test Resultscorrosion-resistance 2000 hours with no signs of corrosion in scribed“x” or on face of article adhesion and flexibility pass, with no coatingloss or cracking abrasion-resistance pass, 0.001 inch coating thicknessloss for 300 liters of sand hydrolytic stability pass, with no coatingloss or cracking

The following examples describe coating compositions which can be usedin the practice of the present invention. In all of the examples herein,the average particle size of the aluminum powder is about 4.5 micronsand the pH of each of the exemplary compositions is within the range of10 to 14.

Example No. 2 below comprises a coating composition which includesphosphate.

Example No. 2

10 g of aqueous solution of sodium silicate, “O” (The PQ Corporation)

9 g of H₂O

0.13 g of tributyl phosphate

0.06 g of silicone wetting aid (to help emulsify the tributyl phosphatein water)

14.4 g of aluminum powder

The next example comprises a coating composition which also includesphosphate (but from a different source than the phosphate used in thecomposition of Example No. 2).

Example No. 3

10 g of aqueous solution of sodium silicate, “STAR” (The PQ Corporation)

9.65 g of H₂O

0.25 g of sodium pyrophosphate

aluminum powder at a ratio of 9 g to 10 ml of above ingredients

The next example comprises a coating composition which includes amixture of sodium and lithium silicates and which also containsphosphate.

Example No. 4

5 g of aqueous solution of lithium polysilicate, “48” (DuPont)

5 g of aqueous solution of sodium silicate, “STAR” (The PQ Corporation)

10 g of H₂O

0.5 g of sodium pyrophosphate

aluminum powder at a ratio of 9 g to 10 ml of above ingredients

The next example comprises a coating composition which includes awetting agent and a mixture of sodium and lithium silicates.

Example No. 5

5 g of aqueous solution of sodium silicate, “STAR” (The PQ Corporation)

10 g of water

5 g of aqueous solution of lithium silicate, “48” (Dupont)

Aluminum powder in an amount of 16.2 g was added to 18 ml of the aboveliquid composition and 0.27 g of Coat-O-Sil 1211 silane wetting agentwas added also and the composition was then mixed. The resultingcomposition was well suited for application by spraying.

In the examples which follow, “%” means weight percent. The next exampleshows the use of a coating composition which contains an organic solventand the use of the composition to form a multi-ply coating.

Example No. 6

16.9% of sodium silicate, “STAR” (The PQ Corporation)

14.2% of lithium silicate, “Ludox Lithium Silicate” (Grace Davidson Co.)

19.5% of H₂O

5.4% of dipropylene glycol n-butyl ether—organic solvent

44.0% of aluminum powder, “ATA 105” from Toyal America

The source (STAR) of the sodium silicate comprised 10.6% Na₂O, 26.5%SiO₂, and 62.9% water. The source (Ludox, previously sold as “48Dupont”) of lithium silicate comprised 2.1% Li₂O, 20% SiO₂, and 77.9%water. A mixture of the sodium silicate, lithium silicate, and water wasstirred for 5 minutes to completely mix the silicates. Next, thedipropylene glycol n-butyl ether was added to the mixture with stirringand the resulting mixture was stirred for another 5 minutes. Thealuminum powder was then added to the mixture and stirred therein forabout 10 minutes to form an aqueous silicate solution having disperseduniformly therein aluminum powder. The resulting coating composition wasapplied to 1010 steel panels (3″×5″×0.03″) by spray application withconventional air-atomizing paint spray equipment until a uniform layerof wet coating was obtained.

The coating was allowed to air dry at ambient conditions (24° C. and 50%R.H.) for a minimum of one hour. A second coat of the coatingcomposition was then applied to form a uniform overlying layer of wetcoating. The multi-ply coating was allowed to dry to the touch atambient conditions and was then placed in an oven at 175° F. for 20minutes, followed by heating for 30 minutes at 650° F. Curing of themulti-ply coating resulted in a solid coating which had a thickness of2.4 mils and which was determined to be non-conductive in that it had anohm reading of greater than 20 ohms. The cured multi-ply coating wasthen made electrically conductive in the same manner as the coating ofExample No. 1. Property Evaluated Test Results corrosion-resistance 1000hours with no signs of corrosion in scribed “x” or on face of articleand no blisters (ASTM B-117)

The next example shows the use of a coating composition which contains asilane and the use of the composition to form a multi-ply coating.

Example No. 7

16.2% of sodium silicate, “STAR” (The PQ Corporation)

13.6% of lithium silicate, “Ludox Lithium Silicate” (Grace Davidson Co.)

22.6% of H₂O

1.5% of N-beta-(aminoethyl)-ganmma-aminopropyltrimethoxysilane,“Silquest A-1123” (OSi Specialties, Inc.)

46.7% of aluminum powder, “ATA 105” from Toyal America

The description of the sodium silicate and lithium silicate is givenabove in Example No. 6. The mixture of sodium silicate, lithiumsilicate, and water was stirred for 5 minutes to completely mix thesilicates. Next, the gamma-aminopropyltriethoxysilane was added to themixture with stirring and stirring was continued for another 5 minutes.The aluminum powder was then added to the mixture and stirred thereinfor about 10 minutes to form an aqueous silicate solution havingdispersed uniformly therein aluminum powder. The resulting coatingcomposition was applied to 1010 steel panels (3″×5″×0.03″) by sprayapplication with conventional air-atomizing paint spray equipment untila uniform layer of wet coating was obtained.

The coating was allowed to air dry at ambient conditions (24° C. and 50%R.H.) for a minimum of one hour. A second coat of the coatingcomposition was then applied to form a uniform overlying layer of wetcoating. The multi-ply coating was allowed to dry to the touch atambient conditions and was then placed in an oven at 175° F. for 20minutes, followed by heating for 30 minutes at 650° F. Curing of themulti-ply coating resulted in a solid coating which had a thickness of2.4 mils and which was determined to be non-conductive in that it had anohm reading of greater than 20 ohms. The cured coating was then madeelectrically conductive in the same manner as the coating of ExampleNo. 1. Property Evaluated Test Results corrosion-resistance 1000 hourswith no signs of corrosion in scribed “x” or on face of article and noblisters (ASTM B-117)

The next example describes a coating composition which includes a silanedifferent from the silane used in Example No. 7.

Example No. 8

16.3% of aqueous solution of sodium silicate, “N” (The PQ Corporation)

13.0% of aqueous solution of lithium silicate, “48” (Dupont, now Ludox)

22.8% of H₂O

2.5% of Silquest A187 (gamma-glycidoxypropyltrimethoxysilane)

45.4% of aluminum powder

Particularly preferred coating compositions are described below inExample Nos. 9 and 10, each of which contains a preferred wetting agent.

Example No. 9

16.9% of aqueous solution of sodium silicate, “N” (The PQ Corporation)

13.5% of aqueous solution of lithium silicate, “48” (Dupont, now Ludox)

23.7% of H₂O

0.5% of polyether modified poly-dimethyl-siloxane wetting agent, “bYK348” (BYK Chemie)

45.4% aluminum powder

Example No. 10

17.4% of aqueous solution of sodium silicate, “STAR” (The PQCorporation)

14.5% of aqueous solution of lithium silicate, “48” of (Dupont, nowLudox)

19.9% of H₂O

2.64% of dipropylene glycol n-butyl ether solvent

0.36% of polyether modified poly-dimethyl-siloxane wetting agent, “bYK348” (BYK Chemie)

45.2% of aluminum powder

It should be appreciated from the above description that the presentinvention provides an environmentally compatible composition that iscapable of forming in a convenient fashion a highly corrosion-resistantcoating that protects underlying substrates from being degraded evenunder the most severe of conditions, for example, those encountered inthe operation of turbine engines.

1. A coating process comprising: (A) applying to a surface a coatingcomposition consisting essentially of an alkali metal silicate and anaqueous liquid phase having dispersed therein solid aluminum particlesto form on the surface a wet coating; and (B) drying said wet coating:(i) under conditions which convert said wet coating to an electricallyconductive, corrosion-resistant, solid coating; or (ii) under conditionswhich form a solid coating which is not electrically conductive(non-conductive) and thereafter treating said non-conductive coatingunder conditions which convert said non-conductive coating to anelectrically conductive, corrosion-resistant coating.
 2. A processaccording to claim 1 wherein the surface is metallic.
 3. A processaccording to claim 2 wherein said wet coating is dried under saidconditions of (i).
 4. A process according to claim 2 wherein said wetcoating is dried under said conditions of (ii).
 5. A process accordingto claim 4 including burnishing the non-conductive coating for asufficient period of time to convert it to a conductive coating.
 6. Aprocess according to claim 2 wherein the coating composition is appliedto the metallic surface of a part of a turbine engine.
 7. A metallic orceramic surface coated with an electrically conductive,aluminum-containing silicate coating.
 8. A metallic surface according toclaim 7 wherein the coating has a thickness of about 0.8 mil to about3.5 mils and corrosion-resistant properties characterized by no greaterthan about 1.6 mm loss of adhesion at scribe when subjected to 5%neutral salt spray at 95° F. for about 1000 hours according to ASTMB-117.
 9. A metallic surface according to claim 7 wherein said coatinghas heat-resistant properties characterized by its being substantiallyfree of cracks, checks, and blisters when the surface is subjected tothe following conditions: heat treatment for 23 hours at a temperatureof about 700° F., followed by heat treatment for 4 hours at atemperature of about 1075° F.
 10. A metallic surface according to claim7 wherein said coating has flexibility properties characterized by itsbeing substantially free of flaking or loosening when subjected to thefollowing conditions: bending a panel coated with the coating through anangle of 90° around a ¼ inch diameter mandrel.
 11. A metallic surfaceaccording to claim 7 wherein said coating has hydraulic oil-resistantproperties characterized by its being free of peeling, blistering, orsoftening when the part is subjected to the following conditions:immersion in Mil-L-7808 oil for 8 hours at a temperature of about 400°F.
 12. A process for converting a solid silicate coating which containsaluminum particles, which is adhered to a surface, and which is notelectrically conductive (non-conductive) to a conductive coating by: (A)subjecting the non-conductive coating to conditions which effectexpansion of the aluminum particles to place them into intimate contactwith one another to the extent that the coating is rendered electricallyconductive; or (B) subjecting the non-conductive coating to a forcewhich is sufficient to compress the particles into more intimate contactwith one another to the extent that the coating is rendered electricallyconductive.
 13. A coating composition which is effective in forming on ametallic or ceramic surface a corrosion-resistant coating and whichconsists essentially of (a) an alkali metal silicate, (b) an aqueousliquid phase having dispersed therein solid aluminum particles, and (c)an additive which is effective in improving the corrosion-resistance ofthe coating and which is selected from the group consisting of (i) anorganic solvent which is partially miscible or immiscible in water; (ii)an organofunctional silane, and (iii) a mixture of said additives.
 14. Acomposition according to claim 13 wherein the additive is an organicsolvent which has a miscibility in water of about 1 ml to about 20 ml ofsolvent per 100 ml of water at about 20° C.
 15. A composition accordingto claim 14 wherein the solvent has a miscibility in water of up toabout 10 ml.
 16. A composition according to claim 15 wherein the solventhas a miscibility in water of up to about 5 ml.
 17. An aqueous coatingcomposition which is effective in forming a corrosion-resistant coatingon a metallic or ceramic surface and which consists essentially ofaluminum particles dispersed in the composition and a mixture of sodiumsilicate and lithium silicate, the total silicate content of thecomposition being about 2.5 wt. % to about 30 wt. % and the weight ratioof sodium silicate to lithium silicate being about 0.25 to 1 to about 4to
 1. 18. A composition according to claim 17 wherein the total silicatecontent of the composition is about 7 wt. % to about 13 wt. %.
 19. Aprocess for forming a multi-ply coating on a metallic or ceramic surfaceby applying thereto an aqueous coating composition consistingessentially of an alkali metal silicate and having dispersed thereinsolid aluminum particles in which (A) the composition is applied to thesurface to form thereon a layer of wet coating; and (B) the layer of wetcoating is air-dried; (C) the composition is applied to the surface ofthe air-dried coating to form thereon an overlying layer of wet coating;and (D) said overlying layer of wet coating is (i) dried underconditions which convert said wet coating to an electrically conductive,solid corrosion-resistant multi-ply coating or (ii) said wet coating isdried under conditions which form a solid multi-ply coating which is notelectrically conductive (non-conductive) and said non-conductivemulti-ply coating is thereafter treated under conditions which convertsaid non-conductive coating to an electrically conductive,corrosion-resistant, multi-ply coating.